A distributed feedback (DFB) laser diode integrated electro-absorption (EA) modulator is important as a light source for transmitting high speed modulated signals (for example, 2.5 to 10 gigabit/sec) over a long distance of more than 80 km. With this integrated light source, if the light is greatly reflected by the end facet of the modulator, through which end facet the light is emitted, then the fluctuation of the wavelength of the oscillated light (chirping) of the DFB laser occurs due to the effect of the reflected light to the DFB laser when the light is modulated. The chirping causes a remarkable degradation of transmission performance in the long distance transmission. Therefore, the end facet of the modulator needs to be made with a low reflection structure.
In order to achieve this low reflection structure, at the end of the modulator, a window structure in which the bandgap energy is greater than the energy of the guided light is adopted as reported by Aoki et al. in IEEE Journal of Quantum Electronics, Vol. 29, No. 6, June 1993, pp. 2088-2096.
In a DFB laser diode integrated electro-absorption modulator (first conventional device) reported by Aoki et al. in the 1993 autumn conference C-96 of the Institute of Electronics, Information and Communication Engineers, a method is adopted in which, after a multiple quantum well (MQW) waveguide layer in the window structure area is removed by etching, InP is buried for growth to realize a window structure. Hereinunder, with reference to FIGS. 1A to 1E, the first conventional device is described.
As shown in FIG. 1, a pair of SiO.sub.2 masks 92 having a width of several tens of .mu.m to several hundreds of .mu.m are formed on an n-InP substrate 91 only at the laser diode region with a gap of several tens of .mu.m. The masks are used for blocking the epitaxial growth.
Subsequently, as shown in FIG. 2, a light guiding layer 93, an active layer 94, a clad layer 95 and a cap layer 96 are sequentially grown according to the MOVPE selective growth process.
Next, as shown in FIG. 3, SiO.sub.2 masks 97 are formed and both of the laser diode region and the modulator region are mesa etched using the SiO.sub.2 masks to form an optical waveguide of 1.5 to 2.0 .mu.m wide. At this time, a portion of 40 .mu.m length between the DFB laser diode and the EA modulator and a modulator side end portion of 20 .mu.m length are etched.
Subsequently, a Fe doped InP layer 98 which serves as a high resistance layer is grown using the SiO.sub.2 masks 97 to bury the optical waveguide. FIGS. 4 and 5 each show a cross-section taken along 4-4' and 5-5' respectively of FIG. 3 after being buried.
In order to realize a low reflectivity at the end surface with good reliability and to maintain an excellent coupling efficiency with the optical fiber, it is necessary to strictly control the length of the window structure region. However, in the first conventional device, since the window region is formed by etching the MQW layer followed by burying or regrowth of the Fe doped InP, variance of the length of the window region is great between devices, and it is difficult to make an excellent window structure having excellent performances with good reliability and good yield.
On the other hand, a DFB laser diode/optical modulator integrated light source (second conventional device) reported by Kato et al. in the 1994 vernal congress C-226 of the Institute of Electronics, Information and Communication Engineers and by Kato et al. in Electronics Letters, 16th, Jan., 1992, Vol. 28, No. 2, pp 153-154 is fabricated by using a growth blocking mask previously formed in a region which will be made to the window structure so that no MQW structure is grown in this region. Therefore, since the length of the window structure is determined with an accuracy of the mask patterning, even if the ordinary photoresist process is used, it can be realized with an accuracy of below 1 .mu.m. Hereinunder, with reference to FIGS. 6 to 8 and 9 to 11, the second conventional device is described.
As shown in FIGS. 6 to 8, after a diffraction grating 100 is formed on an n-InP substrate 101 at a DFB laser diode (DFB-LD) region, a SiO.sub.2 mask 102 is formed. FIG. 8 is a plan view of the mask 102. FIGS. 6 and 7 each show a cross-section corresponding to a cross-section taken along 6-6' and 7-7' respectively of FIG. 8. The mask width is 17 .mu.m at the DFB-LD region and 8 .mu.m at the modulator (MOD) region and the window (WIND) region. A gap NG having a width of 1.5 to 2.0 .mu.m is provided at the DFB-LD region and the MOD region. Further, no gap is present at the region of 25 .mu.m length which will be made to the WIND region. A core layer comprising InGaAsP light guiding layer 103, InGaAs/InGaAsP multiple quantum well (MQW) active layer 104 and p-InP layer 105 is formed on the susbtrate 101 at an area corresponding to the gap NG according to the MOVPE selective growth process.
Next, as shown in FIGS. 9 to 11, the gap WG having a width of 5 to 6 .mu.m is formed in the mask 102 as extending through the DFB-LD, MOD and WIND regions, that is, the gap NG is widened at the DFB-LD and MOD regions and an additional gap is formed at the WIND region. FIG. 11 is a plan view of the mask 102 having the gap WG. FIGS. 9 and 10 each show a cross-section corresponding to a cross-section taken along 9-9' and 10-10' respectively of FIG. 11. An p-InP clad layer 106 and p-InGaAs cap layer 107 are formed on the substrate 101 so as to cover the core layer according to the MOVPE selective growth process. Then, an electrode forming process is carried out to obtain a DFB-LD/optical modulator integrated light source.
In this second conventional device, since the formation of the window structure does not require the process of etching the semiconductor, it is possible to strictly control the length of the window structure region to obtain the device with good controllability and reliability and with high yield. However, as shown in the cross-sectional view of FIG. 9, a level difference occurs on the upper surface of the p-InP clad layer 106 between the modulator region and the window region. This is because since there is no light guiding layer 103, MQW active layer 104 and p-InP layer 105 in the window region, difference in volume thereof between two regions appears itself as such. As shown in FIG. 12, if the level difference occurs on the upper surface of the clad layer 106, since part of the light is reflected by the level difference portion, the window structure does not function as the low reflection structure.